1. Field of the Invention
The present invention generally relates to optical communication technology, and particularly relates to a repeater and a repeating method for a WDM (Wavelength Division Multiplexing) optical communication system.
2. Description of the Related Art
In the technical field of optical communications, there have been growing demands for a wider variety of services and higher speed data transmission. In these years, multiple wavelength optical transmission systems of a transmission speed higher than 40 Gbps per wave are coming closer to realization. Unlike recently popular optical signal transmission of about 10 Gbps, a standard scheme for high-speed optical signal transmission such as 40 Gbps has not yet been determined. In high-speed optical signal transmission of 10 Gbps or higher, not only is signal quality lowered due to optical noise in optical amplifiers, but optical signal properties are also lowered due to wavelength dispersion, nonlinear effects, polarization mode dispersion, spectrum narrowing by optical filters, etc., in transmission lines. In view of these problems, modulation schemes for narrow spectral width that improve dispersion tolerance, nonlinear tolerance, etc., are being researched and developed. Especially, for multiple wavelength optical transmission systems of 40 Gbps, not only systems using NRZ but also systems using modulation schemes such as CSRZ, DPSK, and DQPSK are being proposed. Thus, systems using various transmission schemes are expected to be introduced in the future. The recently popular WDM transmission systems of 10 Gbps are presented in, for example, Non-patent Document 1.    <Non-Patent Document 1> K. Nakamura et al., “1.28 Tbit/s Transmission over 1680 km Standard SMF with 120 km Optical Repeater Spacing Employing Distributed Raman Amplification”, OECC2000, PD1-7
If optical signal transmission schemes differ, properties of optical signals such as noise tolerance, dispersion tolerance, and nonlinear tolerance also differ. This indicates that different transmission schemes require different compensation systems. A direct method for transmitting optical signals of different transmission schemes is to prepare a transmission line (optical fiber) for each transmission scheme so as to build an optical transmission system for each transmission scheme. For example, a transmission system dedicated for optical signals of 10 Gbps and a transmission system dedicated for optical signals of 40 Gbps are separately prepared and separately managed. However, devices for performing optical amplification, optical dispersion compensation and the like for the optical signals of 40 Gbps are expensive. Moreover, a demand for high-speed transmission services of 40 Gbps as high as a demand for the popular optical transmission services of 10 Gbps might not be expected. If the demand for services of the dedicated system is low, unit costs may be further increased.
Problems of facility investment and high costs may be solved by allocating some of the channels of the WDM transmission system of 10 Gbps (e.g. five channels out of forty channels in total) as channels for 40 Gbps. However, there is no established technique for making appropriate adjustments, in repeater devices provided on a transmission line, on channels of different transmission schemes propagating in physically the same transmission line.